This invention relates generally to the field of digitally controlled printing devices, and in particular to continuous inkjet printers wherein a liquid ink stream breaks into droplets, some of which are selectively deflected.
Continuous inkjet printing, uses a pressurized ink source that produces a continuous stream of ink droplets. Conventional continuous inkjet printers utilize electrostatic charging devices that are placed close to the point where a filament of ink breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes. When no printing is desired, the ink droplets are directed into an ink-capturing mechanism (often referred to as a catcher, interceptor, or gutter). When printing is desired, the ink droplets are directed to strike a print media.
Typically, continuous inkjet printing devices are faster than drop-on-demand devices and produce higher quality printed images and graphics. However, each color printed requires an individual droplet formation, deflection, and capturing system.
U.S. Pat. No. 1,941,001, issued to Hansell on Dec. 26, 1933, and U.S. Pat. No. 3,373,437 issued to Sweet et al. on Mar. 12, 1968, each disclose an array of continuous inkjet nozzles wherein ink droplets to be printed are selectively charged and deflected towards the recording medium. This technique is known as binary deflection continuous inkjet.
U.S. Pat. No. 3,416,153, issued to Hertz et al. on Dec. 10, 1968, discloses a method of achieving variable optical density of printed spots in continuous inkjet printing using the electrostatic dispersion of a charged droplet stream to modulate the number of droplets which pass through a small aperture.
U.S. Pat. No. 3,878,519, issued to Eaton on Apr. 15, 1975, discloses a method and apparatus for synchronizing droplet formation in a liquid stream using electrostatic deflection by a charging tunnel and deflection plates.
U.S. Pat. No. 4,346,387, issued to Hertz on Aug. 24, 1982, discloses a method and apparatus for controlling the electric charge on droplets formed by the breaking up of a pressurized liquid stream at a droplet formation point located within the electric field having an electric potential gradient. Droplet formation is effected at a point in the field corresponding to the desired predetermined charge to be placed on the droplets at the point of their formation. In addition to charging tunnels, deflection plates are used to actually deflect droplets.
U.S. Pat. No. 4,638,328, issued to Drake et al. on Jan. 20, 1987, discloses a continuous inkjet printhead that utilizes constant thermal pulses to agitate ink streams admitted through a plurality of nozzles in order to break up the ink streams into droplets at a fixed distance from the nozzles. At this point, the droplets are individually charged by a charging electrode, and subsequently deflected using deflection plates positioned in the droplet path.
As conventional continuous inkjet printers utilize electrostatic charging devices and deflector plates, they require many components and large spatial volumes to operate. This results in continuous inkjet printheads and printers that are complicated, have high energy requirements, are difficult to manufacture, and are difficult to control.
U.S. Pat. No. 3,709,432, issued to Robertson on Jan. 9, 1973, discloses a method and apparatus for stimulating a filament of working fluid causing the working fluid to break up into uniform spaced ink droplets through the use of transducers. The lengths of the filaments, before they break up into ink droplets, are regulated by controlling the stimulation energy supplied to the transducers. High amplitude stimulation causes short filaments and low amplitude stimulations causes longer filaments. A flow of air is generated across the paths of the fluid at a point intermediate to the ends of the long and short filaments. The air flow affects the trajectories of the filaments before they break up into droplets, more than it affects the trajectories of the ink droplets themselves. By controlling the lengths of the filaments, the trajectories of the ink droplets can be controlled, or switched from one path to another. As such, some ink droplets may be directed into a catcher while allowing other ink droplets to be applied to a receiving member.
While this method does not rely on electrostatic means to affect the trajectory of droplets, it does rely on the precise control of the break up points of the filaments and the placement of the air flow intermediate to these break up points. Such a system is difficult to control and to manufacture. Furthermore, the physical separation or amount of discrimination between the two droplet paths is small, further adding to the difficulty of control and manufacture.
U.S. Pat. No. 4,190,844, issued to Taylor on Feb. 26, 1980, discloses a continuous inkjet printer having a first pneumatic deflector for deflecting non-printed ink droplets to a catcher and a second pneumatic deflector for oscillating printed ink droplets. A printhead supplies a filament of working fluid that breaks into individual ink droplets. The ink droplets are then selectively deflected by a first pneumatic deflector, a second pneumatic deflector, or both. The first pneumatic deflector is an xe2x80x9cON/OFFxe2x80x9d type having a diaphragm that either opens or closes a nozzle depending on one of two distinct electrical signals received from a central control unit. This determines whether the ink droplet is printed or not printed. The second pneumatic deflector is a continuous type having a diaphragm that varies the amount that a nozzle is open, depending on a varying electrical signal received by the central control unit. This second pneumatic deflector oscillates printed ink droplets so that characters may be printed one character at a time. If only the first pneumatic deflector is used, characters are created one line at a time, as a result of repeated traverses of the printhead and ink build up.
While this method does not rely on electrostatic means to affect the trajectory of droplets, it does rely on the precise control and timing of the first (xe2x80x9cON/OFFxe2x80x9d) pneumatic deflector to create printed and non-printed ink droplets. Such a system is difficult to manufacture and accurately control, resulting in at least a similar ink droplet build up as discussed above. Furthermore, the physical separation or amount of discrimination between the two droplet paths is erratic, due to the precise timing requirements, therefore, increasing the difficulty of controlling printed and non-printed ink droplets and resulting in poor ink droplet trajectory control.
Additionally, using two pneumatic deflectors complicates construction of the printhead and requires more components. The additional components and complicated structure require large spatial volumes between the printhead and the media, thereby, increasing the ink droplet trajectory distance. Increasing the distance of the droplet trajectory decreases droplet placement accuracy and affects the print image quality. Again, there is a need to minimize the distance that the droplet must travel before striking the print media in order to insure high quality images.
U.S. Pat. No. 6,079,821, issued to Chwalek et al. on Jun. 27, 2000, discloses a continuous inkjet printer that uses actuation of asymmetric heaters to create individual ink droplets from a filament of working fluid and to deflect those ink droplets. A printhead includes a pressurized ink source and an asymmetric heater operable to form printed ink droplets and non-printed ink droplets. Printed ink droplets flow along a printed ink droplet path ultimately striking a receiving medium, while non-printed ink droplets flow along a non-printed ink droplet path ultimately striking a catcher surface. Non-printed ink droplets are recycled or disposed of through an ink removal channel formed in the catcher. While the inkjet printer disclosed in Chwalek et al. works extremely well for its intended purpose, it is best adapted for use with inks that have a large viscosity change with temperature.
Each of the above-described inkjet printing systems has advantages and disadvantages. However, printheads which are low-power and low-voltage in operation will be advantaged in the marketplace, especially in page-width arrays. U.S. patent application Ser. No. 09/750,946, filed Dec. 28, 2000 by D. L. Jeanmaire et al. and U.S. patent application Ser. No. 09/751,232, filed Dec. 28, 2000 by D. L. Jeanmaire et al., disclose continuous inkjet printing wherein nozzle heaters are selectively actuated at a plurality of frequencies to create the stream of ink droplets having the plurality of volumes. A gas stream provides a force separating droplets into printing and non-printing paths according to droplet volume. While this process consumes little power, and is suitable for printing with a wide range of inks, when implemented in a page-width array, a correspondingly wide laminar gas flow is required. The wide laminar gas flow is often difficult to obtain due to the mechanical tolerances involved in the gas flow plenum, with the result that the gas velocity varies somewhat across the printhead, and turbulent flow regions may exist. Non-uniform gas flow has an adverse effect upon droplet placement on the print medium, and therefore image quality is compromised.
It can be seen that there is a need to improve gas-flow uniformity in the design of large nozzle-count printheads such as those used in inkjet printers having page-width arrays.
The above need is met according to the present invention by providing an inkjet printhead, that includes a plurality of nozzle bores from which streams of ink droplets having selectable first and second volumes are emitted; a droplet deflector for deflecting the ink droplets having first and second volumes into first and second paths respectively, the droplet deflector producing a corresponding plurality of physically separate streams of gas, each stream of gas directed on a corresponding one of the streams of ink droplets; and an ink gutter positioned to catch the ink droplets moving along one of the first or second paths.
Additionally, the present invention provides a method for selectively controlling ink droplets in an inkjet printhead, which includes the steps of: emitting streams of ink droplets having selectable first and second volumes; deflecting the ink droplets having first and second volumes into first and second paths, respectively; providing a plurality of separate streams of gas; directing each of the plurality of separate streams of gas at a corresponding one of the streams of ink droplets to move the streams of ink droplets along the first and second paths; and catching the ink droplets moving along one of the first or second paths in an ink gutter.